Inspection system for optical components

Abstract: A cuvette (16) for holding and locating an optical component in position is disclosed. The cuvette (16) comprises a bottom portion having a concave curved inner surface (31) which surface has a radius of curvature equal to or greater than the radius of curvature of the particular surface of the optical component which is intended to be closest to the cuvette's concave inner surface in use.

Description: FIELD OF THE INVENTION This invention relates to a cuvette for holding and locating an optical component. The optical components to be inspected may include ocular optical components such as contact lenses, eyeglasses, intraocular lenses and the like. The present application is divided from European patent application 94924481.8 (0711409). BACKGROUND OF THE INVENTION Previously, optical components such as contact lenses were often inspected manually with the use of a projection-type device such as an optical comparator. Manual inspection systems requiring human intervention are not practical for high-speed production because they are too slow, because human inspectors are prone to making biased judgments, and because inspection results among different inspectors are not uniform. A principal obstacle to automatic inspection has been the inability to produce high contrast images of optical components, such as contact lenses, so that features such as cuts, edges, scratches, tears and chips could be readily detected and measured. In particular, it has been difficult to obtain high contrast images of entire optical components. As used here, the term "features" includes both beneficial features such as certain topographical characteristics of toric lenses and lens boundaries, as well as detrimental features such as scratches, tears and chips. The principal difficulty in obtaining high contrast images of optical components such as contact lenses and eyeglasses is that they are transparent. Moreover, in the case of certain optical components, such as hydrated contact lenses called "hydrogels" which must remain immersed in a fluid such as a saline solution, the production of high contrast images involves a further complication. That is, the refractive indices for the optical component and the liquid solution may be so similar that boundaries between the two are nearly invisible. Images of the optical component are therefore of low contrast. Another difficulty in examining hydrogels is that they cannot be kept in a fixed position during examination and will move distances greater than the size of a small feature. Therefore, it is important to obtain a high contrast image of an entire hydrogel so that an image can be captured by an imaging system in a fraction of a second. The problem sought to be overcome by the present invention concerns the problem of holding an optical component in position for inspection without human intervention. Indeed, in an automated system it may be critical to have a positioning device that is actually part of the optical system used to illuminate the optical component during the automated inspection. EP 0604173 discloses an apparatus for carrying an ophthalmic lens, particularly while the lens is being inspected, including a base member and a well connected to the base member. The well is substantially transparent, and includes a frustoconical side wall having a constant slope, and a hemispherically shaped bottom portion connected to and extending downward from the side wall. The bottom portion of the well has curvature that is about 10% larger than the radius of the curvature of the ophthalmic lens. SUMMARY OF THE INVENTION The present invention provides a novel receptacle or "cuvette" for holding and locating an optical component in position during inspection. The cuvette of the present invention comprises a bottom portion having a concave curved inner surface, wherein the concave inner surface has a radius of curvature that is equal to or greater than the radius of curvature of the particular surface of the optical component under inspection that is intended to be closest to the cuvette's concave inner surface, to thus utilise the force of gravity to position the optical component and minimise its movement; said bottom portion having a convex outer surface, the radius of curvature of said outer surface being selected to that said concave inner surface and said convex outer surface co-operate to provide optical power that will focus light through said bottom portion and to a selected location. Preferably the bottom portion of the cuvette is made of a transparent material. The bottom portion of the cuvette may be made of plastic. The cuvette may contain liquid. Preferably said radius of curvature of said outer surface is selected so that collimated light directed upwardly through said bottom portion is converged by the optical power of the cuvette such that it passes through said optical component and is focussed at an entrance pupil of an image sensing device. The invention provides a combination of the cuvette and a contact lens in the cuvette as the optical component under inspection. Preferably, in the combination the cuvette contains a liquid through which the light passes, and further wherein the radii of curvature of said inner and outer surfaces and the optical characteristic of the liquid combine so that collimated light directed upwardly through said bottom portion is converged by the optical power of the cuvette such that it passes through said contact lens and is focussed at an entrance pupil of an image sensing device. A detailed description of the present invention is set forth below. However, the embodiments described herein are merely illustrative; further embodiments will be apparent to those having ordinary skill in the art. DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of an inspection system for background interest only showing the configuration of an inspection system employing a collimated holes structure. Neither this figure nor any of the other figures herein is drawn to scale. Figure 2 is a diagram of an inspection system showing the path of the focussed light in the configuration schematically illustrated in Figure 1. This is shown for background interest only. Figure 3a is a top view and Figure 3b is a side view of the collimated holes structure (12) used in Figure 1. These figures are not drawn to scale, and in particular, the number and size of the holes in figure 3a are drawn for general illustration purposes only. Figure 4 is a cross sectional view of the holes (8) of the collimated holes structure (12) depicted in Figure 3a. This is shown for background interest only. Figure 5 is a cross sectional view of the preferred embodiment of the cuvette of the present invention showing several specific measurements. A three dimensional perspective of this cuvette can be generated by rotating the drawing about its vertical axis, V. Figure 6 is a diagram of an inspection showing a configuration wherein means for focussing light comprises a refractive convergent light source. This is shown for background interest only. Figure 7 is a diagram of an inspection system showing a configuration wherein means for focussing light comprises a refractive convergent light source. This is shown for background interest only. Figure 8 is a diagram of an inspection system showing a configuration wherein means for focussing light comprises a refractive divergent light source. This is shown for background interest only. Figure 9 is a diagram of an inspection system showing a configuration wherein means for focussing light comprises a reflective collimated light source. This is shown for background interest only. Figure 10 is a diagram of an inspection system showing a configuration wherein means for focussing light comprises a reflective convergent light source. This is shown for background interest only. Figure 11 is a diagram of an inspection system showing an embodiment wherein means for focussing light comprises a reflective divergent light source. DETAILED DESCRIPTION In the inspection system, means for focussing light comprises a light source which transmits collimated light. In a specific configuration the light source comprises an illuminator and a material having a plurality of parallel holes, preferably a collimated holes structure as described in more detail below. In particular, with reference to Figures 1 and 2, an illuminator (10), such as a STOCKER & YALE Model 13 fluorescent illuminator, provides light to means for collimating light - here a collimated holes structure (12). With reference to Figure 1, a spacing member (14), which will be described in more detail below supports an inspection receptacle or "cuvette" (16) above the collimated holes structure (12). The cuvette (16) contains a saline solution (18) for maintaining hydration of an optical component, here a contact lens (20). In addition to containing the solution (18) and the contact lens (20), the cuvette has been engineered to further focus light which has been collimated by the collimated holes structure (12) through the contact lens (20). That light, in turn, reaches image sensing means for sensing an image of the optical component. The image sensing means has an entrance pupil (21, Fig. 2) and a detector (27, Fig. 2). Preferably, the detector of the image sensing means is a CCD array (27, Fig. 2). In a configuration, the image sensing means comprises a camera, preferably a video camera (22) which may optionally be a SONY XC77RR Charge Coupled Device (CCD) video camera connected to a camera lens (24, Fig. 1) which is preferably a NIKON 60 mm. MICRO-NIKKOR lens having an entrance pupil (21, Fig. 2). In this particular embodiment, the optical features are projected in a "bright" - as opposed to a "dark" -- field. The image can then be transmitted to an electronic imaging system (26, Fig. 1). The Entrance Pupil The entrance pupil of a lens ( e.g., the lens of the image sensing means of the present invention) is understood to be the image of the aperture stop as it is appears from the object side of the lens. In the NIKON 60 mm. MICRONIKKOR lens the aperture stop is the adjustable iris which controls the f-number. Hence, the entrance pupil of the NIKON 60 mm. MICRO-NIKKOR lens is the image of the iris of this lens. Light rays which fail to enter the entrance pupil will not reach the image plane of the image sensing means. Therefore, in order to obtain as bright and uniform an image as possible, it is preferable to not overfill or underfill the entrance pupil. With reference to Figure 2, the collimated light (28), which here is a form of focused light in accordance with the present invention, preferably reaches its focus substantially at the entrance pupil (21) of the image sensing means (22). This can also be described as focusing the light "through" the entrance pupil. Unlike traditional imaging systems, where the illumination is not a focused beam, the aperture stop of the image sensing means may also act as a field stop, severely limiting the field of view if the light source is not properly focused through the entrance pupil. Proper focus is achieved by the combined effects of the optics employed by the light source and the optical properties of the cuvette with solution contained within it. When focused light is used , it is preferred but not required that a unique mapping exists between points on the image and rays from the object such that there is substantially a one-to-one correspondence between points on the image and rays from the object. This relationship increases the contrast in the image. Means for Collimating Light In a specific configuration means for collimating light comprises a disc of opaque material having thousands of small, parallel holes through its flat surfaces. A preferred material is dark glass which is light absorbing. Black glass is highly preferable. With dark glass, the interior walls of the holes will be dark in color and will thus absorb light and reduce internal reflections which cause diverging exit rays. In a configuration, the interior walls of the holes are black and the holes are preferably between 10 to 30 µm. in diameter (assuming a circle of best fit where the hole may have a non-circular shape) in a disc of between 1 to 2 mm. thickness. The aspect ratio of the holes (the ratio of the length of a hole to its diameter) is preferably 30:1 or greater. Means for collimating light may, in a preferred configuration, comprise collimated holes structure (12), Figure 3, of black opaque glass approximately 20 mm in diameter and 2 mm thick. In another configuration, it is estimated that there are 600,000 parallel holes of approximately 20 µm diameters aligned perpendicular to the faces of the disc and penetrating through them. The opaque glass has an optical density of at least 65 Db/mm. The open area of the holes is estimated to be 60% of the surface area of a face. Such a structure is sometimes referred to as a "collimated holes structure". A collimated holes structure is manufactured by Collimated Holes, Inc. of Campbell California - Part No. 781-0009. With reference to Figures 3 and 4, the holes (8) of preferred collimated holes structure (12) have interior walls (9, Fig. 4) that are black in color and made of an opaque, light absorbing material, e.g., black opaque glass. Camera A video camera (22) contains a 2/3 inch CCD array (27) of individual sensors - each of which is capable of transducing light energy into electrical energy. The CCD array has 493 horizontal rows of individual sensors. Each row has 768 individual sensors. Thus the total array contains 378,264 individual sensors --which is approximately 63% of the number of holes used to collimate the light in this embodiment. The camera generates 30 frames or images per second. Thus, in this configuration, the means for collimating light has a greater number of hol than the image sensing means has individual sensors in its array. The camera may be connected to a video monitor to view the image of the optical component or connected to an electronic imaging system (26, Fig. 1) which can analyze automatically image electrical signals received from camera (22). When a video camera is used to image a contact lens without the use of means for focussing light in accordance with this invention, it is very difficult or impossible to detect and measure features such as cuts, edges, scratches, tears or chips. With the use of means for focussing light, e.g. collimated light structure (12), high contrast portions of the image of the optical component correspond to the features in the optical component. Thus, there appears to be a 1:1 correspondence between high contrast areas in an image and features in a contact lens. Electronic Imaging System Another configuration further comprises image sensing means incorporating an electronic imaging system (26, Fig. 1) for analyzing image signals received from the camera (22), wherein the electronic imaging system comprises means for detecting in the image signals an "image property" corresponding to a given feature appearing in the optical component. For instance, where an 8-bit electronic imaging system is used (as for example one containing an EPIX Model 10 Imaging Board), an "image property", as the term is used herein can constitute a value in the range from 0 to 255 which is assigned to each gray level in an image. Without means for focussing light ( e.g. collimated holes structure (12)), differences in gray levels caused by features generally were in the range of about 10 gray levels. Since variations in lighting across the image can be approximately of the same magnitude, accurate feature detection and measurement was not possible. With the use of means for focussing light in accordance with the present invention, high contrast images of features having differences greater than 20 gray levels are readily obtained. This has permitted accurate detection and measurement of features. Illuminator The STOCKER & YALE Model 13 fluorescent illuminator was selected as a preferred illuminator because it contains a 20 Khz ballast which appears to the camera (22) with a 30 frame-per-second frame rate as a flicker-free illuminator and thus prevents the occurrence of video "hum" bars in images. The light also has a low heat content which prevents damage to contact lenses being inspected. The light to be used need not necessarily be light within the visible light spectrum. Other forms of light can include, e.g. , infrared light. Cuvette The present invention provides receptacle or "cuvette" (16) for holding an optical component in position during inspection. Referring to Figure 5, the cuvette (16) of the present invention comprises a bottom portion (30) having a concave inner surface (31) for utilizing the force of gravity to hold an optical component in the center of the cuvette. The concave curve of the inner surface (31) has a radius of curvature in order to maximize the force of gravity upon the optical component positioned therein for the purpose of locating the optical component. (The optical component is not shown in Figure 5). The radius of curvature of the inner surface (31) is preferably equal to or greater than the radius of curvature of the particular surface of the optical component which is intended to be closest to the cuvette's concave inner surface (31). Basically, the radius of curvature of the cuvette's concave inner surface (31) is chosen to be as steep as possible to maximize the centering ability of the cuvette while being shallow enough to maintain center contact with the optical component under inspection. This minimizes the distance that an optical component (20, fig. 1), such as a hydrogel, can move in 1/30 second, the time typically required to freeze an image of an optical component. In order to obtain reliable feature detection, it is highly desirable to restrict the distance a hydrogel can move in one frame time ( i.e. , 1/30 second) to a distance less than the smallest size feature which must be detected. When the cuvette of the present invention is used in connection with the inspection system, the cuvette's bottom portion (30) is made of a transparent material, such as an optically clear polycarbonate or polystyrene plastic, and, under such circumstances, the radius of curvature of the bottom portion's inner surface has a second purpose. This curve (31), in combination with the surface geometry of the outer surface (32) of the cuvette's bottom portion, provides optical power. Specifically, the surface geometry of the bottom portion's outer surface (32) which is convex, is selected so that the light which has been focused substantially fills the entrance pupil of the image sensing means and thus reaches its focus substantially at the entrance pupil. It should be noted here that the optical power provided by the cuvette of the present invention; i.e., the power to focus light so that it substantially fills the entrance pupil of the image sensing, can be used in any embodiment of the system of the present invention, regardless of whether the light source transmits collimated light, convergent light, or divergent light. The transparent material used to form the cuvette's bottom portion (30) can also be made of glass or of an acrylic material such as polymethmethacrylate. Under certain circumstances, such as the inspection of hydrated contact lenses, it is desirable to keep the lens immersed in a liquid solution, especially a saline solution. Under such circumstances, the cuvette of the present invention can be constructed to hold a liquid solution. With reference to Figure 5, this can be accomplished by providing a "water-tight" cuvette wherein side walls (33) of the cuvette's bottom portion (30) extend upward for a sufficient height such that liquid will be contained within the cuvette without leaking or spilling. In constructing the inspection system, it may be desirable to separate the cuvette (16) from collimated holes structure (12). By separating the cuvette (16) from structure (12), it is possible to focus the lens of camera (24) on optical component (20) without bringing the structure (12) into the camera lens's depth-of-field. To obtain a desirable distance between cuvette (16) and structure (12), a hollow cylindrical spacing member (14), shown in Figure 1, can be placed in between these two components. The spacing member (14) has a height of approximately 2" (5.13 cm), an inside diameter of approximately 1.25 (3.2 cm) and an outside diameter of approximately 2" (5.13 cm), and the spacing member is made of dark gray plastic so as to minimize internal reflection. The dimensions provided above and in Figure 5 are the more preferred dimensions. It is believed that satisfactory results will be achieved if they are varied by as much as ±50%, more preferably not more than ±25%, of these preferred values. With regard to the radius R* in Figure 5, the radius of curvature of the bottom portion's outer surface (32), this dimension is preferably 26.0 mm when the bottom portion (30) is polymethylmethacrylate and preferably 22.3 mm when the bottom portion is polycarbonate. R**, the radius of curvature of the bottom portion's inner surface (31), is preferably 12.5 mm. With regard to alignment of this configuration of the system, the image sensing image means is optically aligned with the collimated holes structure (12), the clear cuvette (16) and the optical component to be inspected (20) such that the optical axes of each are coaxial. (See optical axis (29) in Figure 2). USE OF A REFRACTIVE COLLIMATED LIGHT SOURCE In another configuration, means for focussing light comprises a refractive collimated light source, comprising an illuminator (any light source) and a collimating lens. This configuration, which is shown in Figure 6, comprises an illuminator (40), such as the STOCKER & YALE Model 13 illuminator, which provides light to a pinhole (41). A collimating lens (42) is then used to collimate the light. The pinhole (41) is then considered as the object for the collimating lens (42) which focuses the image of the pinhole at infinity. In a specific configuration the pinhole diameter was 500 microns and the collimating lens (42) was an achromatic doublet (P/N 23-9756 from Ealing Electro-Optics of Holliston, Massachusetts) with a focal length of 200 mm. Of course, one of ordinary skill in the art will recognize that the diameter and the focal length may be varied depending upon the luminance of the illuminator, the sensitivity of the image sensing means, and the degree of collimation required to obtain a satisfactorily high contrast image on the image sensing means. The specific pinhole used in this preferred embodiment was Model No. 39729 from Edmund Scientific Corp. of Barrington, New Jersey. Preferably, a cuvette as described above is used in connection with this configuration and the collimated light is focused by the cuvette before passing through the optical component under test. With reference to Figure 6, the curvature of the cuvette radii must be chosen such that the focal length of the cuvette (46) is appropriate to focus the light substantially at the entrance pupil (51) of the image sensing means (52), when the distance between the image sensing means and the optical component is adjusted to provide the desired magnification of the optical component. Preferably, illuminator (40), pinhole (41), collimating lens (42), cuvette (46), solution (48), entrance pupil (51), camera (52) and CCD (53) are optically aligned along optical axis (49). USE OF A REFRACTIVE CONVERGENT LIGHT SOURCE Figure 7 shows an alternate configuration which does not use collimatec light. In this embodiment a converging lens (62) would be used. Light from the illuminator (60) would be made to converge through converging lens (62) by adjusting the distance between the converging lens and the pinhole (61) so that this distance is more than the focal length of the converging lens. Light would then be focused further through the surfaces of the cuvette (66) and solution (68), until it focuses substantially at the entrance pupil (65) of the image sensing means (camera 64). The high contrast properties of the image would be equivalent to the system incorporating collimated light. Preferably, illuminator (60), pinhole (61), converging lens (62), cuvette (66), solution (68), entrance pupil (65), camera (64) and CCD (63) are optically aligned along optical axis (69). USE OF A REFRACTIVE DIVERGENT LIGHT SOURCE In another configuration, light would be focussed by means of refraction light emitted from a divergent light source comprising an illuminator (any light source) and a diverging lens. Specifically, as shown in Figure 8, light from illuminator (70) would be made to diverge through diverging lens (72) by separating pinhole (71) and the diverging lens (72) by less than one focal length of the diverging lens. Light would then be focused further through the surfaces of cuvette (76) and solution (78), until it focuses substantially at the entrance pupil (75) of the image sensing means (camera 74). A high contrast image would thus be achieved similar to the high contrast image previously described. Preferably, illuminator (70), pinhole (71), diverging lens (72), cuvette (76), solution (78), entrance pupil (75) and camera (74) and CCD (73) are optically aligned along optical axis (79). USE OF A REFLECTIVE COLLIMATED LIGHT SOURCE Knowing that light from the illuminator must be substantially focused at the entrance pupil of the image sensing means and the image of the optical component being tested must be focused at the image sensing means, this can be accomplished by a light source using reflective optics as well as refractive optics or a collimated hole structure, as previously described. This configuration comprising a reflective collimated light source is shown in Figure 9. Thus, Figure 9 shows a system identical to Figure 6 except that an off axis parabolic mirror (87) is used as a collimator. The mirror is aligned with the optical axis (89) and there is a pinhole (81) which is placed off-axis at the focal point of the mirror. The pinhole receives light from illuminator (80). Preferably, parabolic mirror (87), cuvette (86), solution (88), entrance pupil (85), camera (84) and CCD array (83) are optically aligned along optical axis (89). USE OF A REFLECTIVE CONVERGENT LIGHT SOURCE Figure 10 is similar to Figure 9 except that the pinhole (91) is more than one focal length distance from the off-axis parabolic mirror (97). The light source is then convergent as in Figure 7. The pinhole (91) receives light from illuminator (90). Preferably, parabolic mirror (97), cuvette (96), solution (98), entrance pupil (95), camera (94) and CCD array (93) are optically aligned along optical axis (99). USE OF A REFLECTIVE DIVERGENT LIGHT SOURCE Figure 11 is similar to Figure 9 except that the pinhole (101) is less than one focal length from the off axis parabolic mirror (107). The light source is then divergent as in Figure 8. The pinhole (101) receives light from illuminator (100). Preferably, parabolic mirror (107), cuvette (106), solution (108), entrance pupil (105), camera (104) and CCD array (103) are optically aligned along optical axis (109).

Claims: A cuvette (16) for holding and locating an optical component in position for inspection comprising: a bottom portion having a concave curved inner surface (31), wherein the concave inner surface (31) has a radius of curvature that is equal to or greater than the radius of curvature of the particular surface of the optical component under inspection that is intended to be closest to the cuvette's concave inner surface (31), to thus utilise the force of gravity to position the optical component and minimise its movement; said bottom portion having a convex outer surface (32), the radius of curvature of said outer surface (32) being selected to that said concave inner surface and said convex outer surface co-operate to provide optical power that will focus light through said bottom portion and to a selected location. The cuvette of claim 1, wherein the bottom portion of the cuvette (16) is made of a transparent material. The cuvette of claim 1, wherein the bottom portion of the cuvette (16) is made of plastic. The cuvette of claim 1, wherein the cuvette (16) is capable of containing liquid. The cuvette of claim 1, wherein said radius of curvature of said outer surface (32) is selected so that collimated light directed upwardly through said bottom portion is converged by the optical power of the cuvette (16) such that it passes through said optical component and is focussed at an entrance pupil of an image sensing device. A combination of the cuvette of claim 1 and a contact lens in the cuvette (16) as the optical component under inspection. The combination of claim 6, wherein the cuvette (16) contains a liquid through which the light passes, and further wherein the radii of curvature of said inner and outer surfaces (31,32) and the optical characteristic of the liquid combine so that collimated light directed upwardly through said bottom portion is converged by the optical power of the cuvette (16) such that it passes through said contact lens and is focussed at an entrance pupil of an image sensing device.

Inventors: EPSTEIN SHELDON L, GORE RICHARD G, EPSTEIN, SHELDON L., GORE, RICHARD G.
Applicants: JESSEN WESLEY CORP, WESLEY JESSEN CORPORATION
Application year: 1994
Main classification: G01M11
IPC codes: G01M11